Operating system interference in parallel programs can cause
tremendous performance degradation. This paper discusses the
interference caused by the quiet generation of subnormal floating
point values. We analyze the performance impact of subnormal values
in a parallel simulation of a stress wave propagating through a
three dimensional bar. The floating-point exception handling
mechanisms of various parallel architectures and operating systems
lead to widely differing performance for the same program. We show
that a parallel program will exhibit greatly amplified performance
degredation due to this interference. In addition we provide an
simple example program that demonstrates underflow on a single
processor. Finally we suggest a novel option for fixing these
undesired slowdowns.